The retroperitoneal space (retroperitoneum) is the anatomical space (sometimes a potential space) in the abdominal cavity behind (retro) the peritoneum. Description from imgarcade.com. I searched for this on bing.com/images
Pneumoretroperitoneum - presence of gas within the retroperitoneal space. Typically the air outlines structures like the kidneys, psoas muscles and retroperitoneal portions of the bowel (duodenum, ascending colon, descending colon and rectum). It is almost always due to perforation of a hollow retroperitoneal viscus such as may occur in duodenal ulcer disease, abdominal trauma, diverticulitis, ischemic colitis, colorectal cancer and following endoscopy.
Psoas muscle abscess and fluid collections are located in the retrofascial space rather than in the retroperitoneal space because the psoas muscles are located posterior to the transversalis fascia, which is the posterior boundary of the retroperitoneum. Ultrasonography is diagnostic in only 60% of cases of psoas abscess, compared with 80% to 100% for CT. Read more: http://radiopaedia.org/articles/psoas-muscle-abscess